Requirements for various kinds of conductive materials used in the field of electric and electronic industry have become severe, and materials which can be made small and light and have long term stability and high performance have been desired.
Highly conductive polymers which satisfy such requirements have been developed extensively in recent years, and many applications of these polymers have been described in, for example, European Pat. No. 203,438 and West German Pat. Nos. 3,409,655 and 3,307,954.
As examples of stable conductive polymers there may be mentioned heterocyclic polymers such as polythiophene, polypyrrole, etc., and not only scientific analysis was directed to such stable conductive polymers, but also many investigations were carried out from the aspect of industrial application. For instance, their use as the electrode materials for secondary cells or the electrochromic materials was proposed.
As the process for producing the conductive polymers there are known a process for chemical polymerization as described in European Pat. No. 182,548 and a process for electrochemical polymerization as described in West German Pat. Nos. 3,533,252 and 183,053, and they are chosen depending on the application of the polymers.
The conductive polymers, however, have low electrical conductivity in a neutral state, so that, in order to obtain a relatively high electrical conductivity of more than 1 S/cm, an electron acceptor (or an electron donor), so-called "dopant", is necessarily added to chemically or electrochemically react with the polymers, as described in West German Pat. No. 3,518,886 and U.S. Pat. Nos. 4,499,007 and 4,657,985.
Various electron acceptors are known, such as protonic acids (e.g., various kinds of halogen, metal halides, p-toluenesulfonic acid, etc.) and oxidizing agents (e.g., tetracyanoquinodimethane, chloranil, etc.). They have relatively low molecular weight and are adapted for use in the conductive polymers to perform reversible oxidation-reduction reaction, so-called "doping and dedoping", such as electrode materials for secondary cells or electrochromic materials. However, they are not suitable for use in the system where stable conductive state is required for a long period of time as in condenser electrodes, transparent conductive films, etc.
Under the circumstance, it has been proposed to use a polymer electrolyte as the dopant. For instance, one process comprises producing polypyrrole by electrochemical polymerization in the presence of a polyanion (e.g., polyvinylsulfuric acid anion, polystyrenesulfonic acid anion, etc.) together with an electrolyte, whereby the polyanion is captured by the conductive polymer, as described in Shimizu et al., the pre-printed papers for the Meeting of High Polymer Society, Vol. 34, No. 10, p. 2829 (1985). It is described that the polyanion captured is almost immobile, so that it is considered that this contributes to the stabilization of the conductive state. According to this process, however, the content of the dopant is limited depending on the polymerization condition and it cannot be freely controlled. Another process is that a film of polyanion is prepared and then a conductive polymer is compounded therewith, for instance, as described in Shimizu et al., the pre-printed papers for the Meeting of High Polymer Society, Vol. 34, No. 10, p. 2825 (1985). This process also has a defect that the content of dopant cannot be freely controlled as in the above-described process.